Complex electronic systems commonly contain a quantity of sub-assemblies on printed circuit boards (PCBs) or other modules which are typically located and interconnected in a rack connected via a back plane, or in an enclosure connected via wire. As the core voltage of integrated circuits has continued to fall to below 3.3 Volts to 2 Volts and even less, power distribution to these sub-assemblies conventionally uses an architecture where each sub-assembly creates its own specific power, using an intermediate system power as an input. This is commonly referred to as a distributed power architecture (DPA) and it a more efficient architecture for getting regulated power to the various sub-assemblies when compared to the centralized power architectures (CPA) of prior assemblies of the past.
In a prior known approach for a DPA, the primary system power supply will receive an input voltage and reduce it to one or more intermediate system voltages to be bussed to each of the PCBs sub-assemblies. Each PCB sub-assembly typically includes a DC-DC converter that will convert the system supply voltage to one (or more) well-regulated supply voltages for the sub-assembly PCB. With the addition of power regulation, the DC-DC converters are commonly referred to as switching regulators (SRs) or point-of-load (PoL) converters. Electrical isolation may be provided by either the primary system power supply or the individual switching regulators. The switching regulator is a commonly used and versatile element allowing the various sub-assembly components to receive the power supply voltages required without requiring adding a specific voltage and buss to the system power architecture. In the case of a high current, low voltage PoL converter, such as one rated at 30 A, 1V, the PoL converter significantly reduces the line losses and cost that would otherwise be needed when distributing a 30 A current by allowing the power to be distributed as a high voltage, low current, such as 3 A, 12V to the various sub-assemblies. This approach allows the power to be distributed in a smaller sized, lower cost and lighter weight wire or buss than would otherwise be needed.
FIG. 1 depicts a distributed power architecture. DPA 100 has an input voltage 101 which feeds the system power supply 103. The system power supply is connected to a power buss 105 which, in this non-limiting, illustrative, example, distributes voltages such as 24V and 12V within the system. The DC-DC switching regulators 107a, 107b . . . 107n located on the sub-assembly PCBs (not shown) are connected to the system power buss 105 and may utilize one or both of the system power supplies. Illustrating the versatility of the onboard switching regulators, regulator 107a has outputs of 10V and 3.3 Vs; regulator 107b has outputs of 1.0V and 1.5V; and regulator 107n has a single output of 8V. In consumer and commercial electronic applications, the input voltage 101 could be 100 Vac to 240 Vac. In telecom applications, for example, where the system is run from standard battery voltages, the input voltage could be between 36 Vdc to 72 Vdc with a 48V DC input being the typical input voltage in known systems.
Progress has been made in improving the efficiency of the two stage DPA including allowing the intermediate voltage to be a semi-unregulated voltage, running in an open loop manner. This approach is disclosed in U.S. Pat. No. 7,187,562, titled “Two Stage Power Conversion Circuit” to Stojcic et. al. (the “562 patent”). As explained in the '562 patent, allowing a semi-unregulated intermediate voltage in the distributed power architecture reduces complexity of the first stage and does not have an adverse effect on the individual switching regulators on the sub-assemblies, since each sub-assembly is configured to regulate the output voltages. For the second stage switching regulators, the multi-phase buck converter has become a commonly used converter architecture for the point-of-load (PoL) converters where high currents and low voltages, such as 1V at 40 A, are required.
FIG. 2 illustrates in a simple schematic a two phase buck converter 200 with a synchronous rectifier, current doubler output stage, and a controller. FIG. 2 illustrates a buck converter 200 which is a rectifier that utilizes transistor switches Q1, Q2 as high side drivers and transistor switches Q3 and Q4 as synchronous rectifiers. The output voltage VOUT is coupled to the switching node through two inductors L3 and L4, which serve to double the output current while halving the voltage (when compared to a buck converter with a single inductor and single phase). The switching controller 201 receives output voltage information in a feedback loop which is used to allow the controller 201 to synchronously switch Q1, Q2 and to use Q3, Q4 to act as synchronous rectifiers and to regulate the output voltage or current.
Regulation modes are divided into two basic types: pulse width modulation (PWM) and hysteretic. Within the PWM type are voltage-mode control (VMC) and current-mode control (CMC) controllers. One skilled in the art will recognize that various off the shelf integrated circuits available from a number of manufacturers implement these various well known converter control modes. For example the SG3524 “Pulse Width IC” commercially available from Microsemi implements VMC, the UC3843 current mode PWM controller manufactured by Unitrode (now available from Texas Instruments Incorporated) implements CMC, and the TPS53632 integrated circuit available from Texas Instruments Incorporated implements hysteretic control.
The control mode selected depends on the requirements of the power supply, however hysteretic control inherently provides lower quiescent current and excels in extreme down conversion applications common in PoL converters today. There are several types of hysteretic control presently in use. The original hysteretic approach provided a very simple circuit with fast regulation but also with a widely varying switching frequency and a variable pulse width. Modified hysteretic control has since been developed to improve converter performance. In one example approach, a semi-fixed duration on-pulse is used to turn on a high side driver switch at each trigger event. By adjusting the duration of the pulse, and triggering the pulses based on the output voltage and on the input voltage, regulation can be achieved. Additionally, hysteretic control achieves good regulation when compared to other approaches and can provide a sufficiently fast voltage and current slew rate to supply processor cores, DSP cores and other low voltage, power hungry ICs. The use of power savings and sleep modes in integrated circuits, coupled with a very fast transition to a full voltage or high speed clocking rate, makes the need for a fast response from the PoL converter critical to system performance when supplying power to these high performance ICs. Hysteretic controllers can provide cycle by cycle control with fast response to changes in output or load conditions, making these controllers popular for providing power supplies to these advanced integrated circuits.
The two-stage approach to DPA is commonly used today because the conversion of a high input voltage (a DC input of 36-75 Volts, for non-limiting examples) to a sub 3.3V level with a single buck converter stage has a typical conversion efficiency in the low 80% range or even worse for some prior systems. As indicated in FIG. 1, a typical efficiency of the first stage system power supply is 95%-98% and a typical efficiency of the second stage switching regulators used in the prior approach converters is 90% to 95%, resulting in an overall system efficiency in the 85% to 93% range when using the two-stage approach. It is well known that for a given power level, the efficiency of the converter typically decreases with decreasing output voltage. For example, a 12V, 30 W switching regulator converter providing 6V @ 5 A would be more efficient than the same converter providing 1V @ 30 A at the output. With that in mind, the typical overall efficiency of a prior known approach low-voltage output PoL converter is normally closer to the 85% end of the overall efficiency range.
Accordingly, due to the continuing efforts to reduce the size of electronic products and to increase efficiency which is necessary to reduce component size without adversely increasing the device operating temperatures, further improvements in the size and efficiency of power converters are needed and desired.